Many RF communication systems introduce distortion into messages transmitted over the communications link and make it difficult to reproduce the original message at the receiver. This distortion can result from multipath reception, group delay distortion, noise amplitude distortion, interference, dispersive facing or the time-spread of the time response. As a result, many communication systems introduce an anti-distortion compensation scheme at the receiver.
Adaptive equalizers, i.e., recursive filters, are often used. In these systems, the characteristics of the channel are estimated, and the data subjected to a pre-distortion filter function based on the observed influence of the channel on the communications signals as they are received. Some systems adapt to the distortion introduced into the data and rely on unknown signal inputs. Other systems provide adaptive equalization to compensate time spread and frequency spread distortion. For example, a known signal is interleaved with segments of the original message at the transmitter to create alternating bursts of unknown data in a known signal spaced apart in time and transmitted to the receiver. Instead of using a time domain equalization approach, the distortion compensation equipment converts the received message signal and a known test signal into the frequency domain for processing to obtain an estimate of the transfer function of the communication link that is used to recreate the originally transmitted message signal. There is some delay factor involved in this type of system.
The use of adaptive equalizers is routine and usually (a) train-off the data, for example, a decision feedback equalizer, (b) train-off a prescribed preamble such as an IEEE 802.11 packet based systems, or (c) train-off embedded ambles contained within the data, such as used in cellular radio and in disclosed commonly assigned U.S. Pat. No. 4,365,338, the disclosure which is hereby incorporated by reference in its entirety.
In the incorporated by reference '338 patent, the signal transmission and data recovery devices use a selected transmission scenario and adaptive filtering process in which the transmitted message is assembled to contain alternately arranged known and unknown pluralities of data symbols. At the transmitter, the known data symbols are generated by a sequence generator, with an identical generator provided at the receiver for reproducing the known symbol sequences and enabling data recovery. At the receiver, a prescribed data recovery algorithm containing a transversal filter function and a data estimate refinement is applied to the communications signal. The algorithm uses a priori knowledge of data symbols of those portions of the transmitted message between which unknown data symbols are located. The transmitted message typically contains a plurality of successive frames, with each frame containing N known data symbols operative as training symbols, followed by M unknown data symbols or vice versa. Synchronization of the local pseudo noise (PN) sequence generator with the PN sequence of which the successive sets of N known data symbols are composed is achieved by using a prescribed tone sequence/initialization scheme.
The use of the powerful forward error correction codes in this type of modern digital transmission systems described above makes training any adaptive equalizer such as disclosed in the incorporated by reference '338 patent difficult at low signal/noise (Eb/No) ratio since the forward error correction (FEC) code exhibits a lower Eb/No performance threshold than the equalizer. Thus, it is desirable to improve the channel estimator performance and improve the signal-to-noise ratio performance of the equalizer training algorithm to an arbitrary low Eb/No value, including a value that is lower in value than the FEC code threshold.